Relaxation measurements in the regime of the second magnetization peak in Nb films

Abstract

We report on magnetic measurements as a function of field, temperature and time (relaxation) in superconducting Nb films of critical temperature Tc = 9.25 K. The magnetic measurements as a function of field exhibited a second magnetization peak (SMP) which in general is accompanied by thermomagnetic instabilities (TMIs). The lines where the SMP occurs and where the first flux jump in the virgin magnetization curves is observed, end at a characteristic point (To,Ho)=(7.2 K,80 Oe). Relaxation measurements showed that for T<To=7.2 K the activation energy Uo and the normalized relaxation rate S exhibit non-monotonic behavior as a function either of temperature or field. The extrema observed in Uo and S coincide with the onset and the maximum points of the SMP. In the regime T>To=7.2 K both Uo and S present a conventional monotonic behavior. These results indicate that the SMP behavior observed in our Nb films is promoted by the anomalous relaxation of the magnetization.Comment: To appear in Physica